The present invention relates to a bacterial infection-preventive agent for fish and shellfish larvae, particularly marine fish and shellfish larvae and, among them, shells such as abalones (ear shells), oysters and scallops for preventing them from the bacterial infection while they are bred, and also to a method for breeding them.
The most popular method for counteracting the bacterial infection of fishes and shellfishes while they are bred in water is the administration of antibacterial agents such as antibiotics. For example, it was proved that some antibiotics have a high bacteriocidal effect on pathogenic bacteria (mainly vibrio) which harm bivalve larvae.
There is another method for breeding them in running water. For example, bivalve larvae are bred in a method wherein water for the larvae is not moved. That is, they are bred in a tank which is once filled with water, without pouring fresh water and without circulation or filtration of the water, and the whole water is changed periodically in order to keep it clean. The running water method is considered to be effective in preventing the propagation (abnormal growth) of the bacteria because fresh water can be always fed.
Although effects of preventing the bacterial infection can be expected to some extent in the both of the above-described methods, these methods still have problems described below.
Although the antibacterial agents have a remarkable effect of killing pathogenic bacteria, they also change the bacterial flora which contributes to the stabilization of the quality of the water in the tank. In such a water tank, bacteria are usually contained in an order of a million per ml of the water. It is known that when an antibiotic is thrown into the water, the number of the bacteria is reduced to about 1/1000 to 1/10,000 in a short time and that 24 hours after stopping the addition of the antibiotic, the number of the bacteria is increased again to the former level. Namely, some kinds of bacteria which are sensitive to the antibiotic are killed and other kinds of bacteria survive and propagate. When the bacterial florae composed of various kinds of bacteria, wherein the bacteria control each other and thereby keep the stable state of the bacteria, are decomposed and only a few kinds of the bacteria propagate, there is a high possibility of the occurrence of undesirable phenomena such as secondary infection and extreme reduction in the activity of the larvae. Another problem occurs in that when the antibacterial agent is used for a long time or repeatedly, drug-resistant bacteria will appear.
On the other hand, the running-water method is suitable for breeding the larvae and it has some merits, but it also has defects. Namely, from the viewpoint of the counteraction against the bacterial infection, the larvae-breeding conditions thereof are unsuitable (as compared with those of the not-moving water method) for the propagation of bacteria. However, when the larvae once suffer from some disease, there is substantially no countermeasure. In other words, since the stable bacterial flora is not formed in the tank in this method, the mutual controlling effect of the bacteria is unexpectable. In addition, the above-mentioned administration of an antibacterial agent is impossible. Only one method for protecting them from death from the mass infection is to keep a high activity of the larvae by continuously feeding water of a high quality so as to realize a high anti-microbial power of the larvae. If bacteria having a very strong pathogenicity propagate and the larvae have started to die, the control will be more difficult than in the not-moving water method.
Thus, in all the treating methods in the prior art, the mechanism of the bacterial infection or the mechanism of the appearance of the pathogenicity was substantially not taken into consideration. Namely, in all the methods, efforts are made only for the purpose of eliminating or reducing the bacteria in the water tank. For finding an effective method for preventing the infection or an effective treating method, not only the bacteria per se but also the pathogen and the mechanism of the infection should be elucidated, and also investigations should be made on the effective method against respective elements which cause the disease. Under these circumstances, the development of an effective method for preventing the bacterial infection of larvae of fishes and shellfishes is demanded.